1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to a mask and a method of fabricating a mask used in extreme ultraviolet (EUV) lithography.
2. Discussion of Related Art
Ongoing improvements in lithography have allowed the shrinkage of semiconductor integrated circuits (IC) to produce devices with higher density and better performance. Deep ultraviolet (DUV) light with a wavelength of 248, 193, 157, or 126 nanometers (nm) may be used for optical lithography. However, a paradigm shift to Next Generation Lithography (NGL) should occur around the 70-nm node.
EUV lithography, a leading candidate for NGL, is based on exposure with EUV light having a wavelength of 10-15 nanometers. EUV light falls within a portion of the electromagnetic spectrum generally known as soft x-ray (2-50 nm). DUV lithography uses transmissive masks made from fused quartz, but nearly all materials are highly absorbing at the EUV wavelength so EUV lithography uses a reflective mask.
An EUV step-and-scan tool typically uses a 4xc3x97-reduction projection system. A wafer is exposed by stepping fields across the wafer and scanning an arc-shaped region of the EUV mask for each field. An EUV step-and-scan tool may have a 0.10 Numerical Aperture (NA) with 4 imaging mirrors. A critical dimension (CD) of 50-70 nm may be achieved with a depth of focus (DOF) of about 1 micrometer.
Alternatively, an EUV step-and-scan tool may have a 0.25 NA with 6 imaging mirrors to print a smaller CD of 20-30 nm, at the expense of a smaller DOF. Other tool designs with a 5xc3x97- or a 6xc3x97-reduction projection system, may also be used for EUV lithography.
Optical inspection of a mask is based on a comparison of the light signals in the patterned regions relative to the non-patterned regions. A high contrast is necessary in order to achieve sufficient sensitivity for defect detection. The transmissive masks used in DUV lithography can be inspected without difficulty since the contrast between the opaque regions and the clear regions is high at UV/DUV wavelengths. However, it is difficult to inspect the reflective masks used in EUV lithography since the contrast between the absorber region and the mirror region is low at UV/DUV wavelengths.
Thus, what is needed is an EUV mask with high contrast at the inspection wavelength and a process for fabricating such an EUV mask.